Sound recording apparatus



April 30, 1935. B. KR uzEiQ 3 7 SOUND RECORDI NG APPARATUS Filed May 25, 1931 :5 Sheets-Sheet 1 INVENTOR BARTON KREUZER ATTORNEY April 30, 1935.

B. KREUZER SOUND RECORDING APPARATUS Filed May 25, 1931 3 Sheets-Sheet 2 B. KREUZER 1,999,700

April 30, 1935.

SOUND RECORDING APPARATUS Filed May 25, 1951 5 SheetsS heet s XXXXXHXYY unnnn INVENTOR BARTON KREUZ EPs BY [K W ATTORNEY Patented Apr. 30, 1-935 SOUND RECORDING APPARATUS Barton Kreuler, Rockaway Park, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 25,1931, Serial No. 539,899

iclaims.

This invention relates to sound recording aD- paratus and a method of operationtherefor, more particularly it relates to photophonographic sound recording apparatus and to an amplifier construction for use in conjunction therewith or in ggniunction with other sound recording appara- When photophonographic records are reproduced there is a tendency for considerable ground I noise to be produced by the portions of least density in such record, and it has been proposed to eliminate such ground noise by masking off the portion of the sound track not occupied by a variable area record or by producing additional ex-- 1| posure in such portions of the variable density In the variable area system, the masking is accomplished by a movable mask magnetically con trolled which screens oil? the unused portion of 20 the sound track or of the slit Qvhich is imaged upon the sound track or is otherwise so located as to perform the same function, while in the variable densitytype of recording .the desired resuit is accomplished by changing the amplitude 25 of a direct current or potential I applied to the light valve; glow lamp or other density determining means.

The position 'of the shutter or the quantity of bias applied or, if such a system is used in record- 30 ing where the meanpath of the record is shifted laterally as the volume changes, then the axis of such path, must in each case be caused to more or less accurately follow the envelope of the sound record curve. At the same time, it is necessary- 35 that such control shall not follow the individual sound frequencies nor must it have such a period of resonance that. it can become resonant to any frequency within the range recorded. It is furthernecessar'y that if the amplitude of the sound 40 increases suddenly, the control member shall follow so promptly as to avoid noticeable distortion or shall even precede such increase in order to avoid cutting oil the peaks of the waves, or in the case of a spiral record, cutting over into adjacent. 45 sound track. Likewise, if the volume of sound de- -'creases, the control member must follow within such a short time that the ground noise timing the interval is not noticeable. I

One object of the present invention is to pro- 50 duce an amplification system and method which will operate the control member in accordance with the above requirements.

Another object of the invention is to provide an amplifier which will provide a voltage or cur- 5 rent at all times proportional to the envelope of (01. run-199.9

the sound record wave independent of the individual frequencies.

Another object of this invention is to provide such an amplifier wherein the control member will be promptly actuated on a sudden increase in 6 sound. 1

Another object of the invention is to provide such an amplifier wherein the transmission of,- the sound impulse to the recording member is delayed until after the control member has had 10 time to function when the sound increases.

Another object of the invention is to provide an amplifier wherein the control member is prevented from functioning on'a sudden decrease in sound until after such decrease is applied to the recording member. Y

Another object of this invention is to produce such an amplifying system wherein the control member will be precluded from operating at any reproducible frequency.

Another object of this invention is to provide an amplifier wherein -a direct current or voltage is provided which is proportional-to the fluctuating potential applied to the input of the amplifier.

Another object of the invention is to provide such an amplifier wherein a sudden increase in the input of fluctuating current willbe promptly followed by a change in the direct current or potential of the output but wherein a sudden decrease in the fluctuating input will be followed at a longer period by a corresponding change in the D. 0. output.

Figure 1 is a schematic diagram of the type of circuit used to control the rate of operation of the amplifier.

Figure 2 is a schematic diagram of one type of amplifier including such circuit. I

Figure 3 is a diagrammatic illustration of the relation of the amplifier of Fig. 2 to the other elements of the circuit and recording apparatus.

Figure 4 is a diagrammatic illustration of a different relation of the apparatus. 5

.Figure5 is a schematic diagram of an interconnection between the amplifier of Fig. 2 and the X main amplifier of a recording apparatus as shown in Fig. 4.

It will be apparent that applicants' amplifier is intended for use in conjunction with soundrecording apparatus of any of the usual electrophotographic or electro-mechanical varieties which include a microphone, a series of thermionic amplifiers, and an electrically-operated recorder, and inasmuch as such amplifiers and recorders are well known in the art they are not herein described.

'the left-hand end of this circuit, either through a transformer as shown or by a direct or a capacitative coupling, it will be rendered uni-directional by the rectifier I. This uni-directional current will flow through the resistance 9, which may be either fixed or variable or may be considered as representing the resistance of the rectifier in the direction of current flow if a rectifier is chosen having an'appropriate value, and will charge the condenser 3, around which is shunted the resistance 2. The grid of the tube 6 will not draw any appreciable current, and that portion of the circuit will therefore not affect the potential built up across the condenser 3, which potential, when in a steady state, is determined by the values of the A. C. input, the resistances 2 and 9, and the capacity 3.

If there is any increase in the A. C. supplied, an increased uni-directional current will flow through the rectifier I and the resistance 9, and there will be a correspondingly increased potential drop across the condenser 3 which will in turn affect the grid of the tube 6. It should be noted that the rate of increase for a given increase of input is determined by three factors,the values of the resistors 9 and 2 and the value of the capacity 3.-

On the other hand, if there is any decrease in the A. C. supplied, a decreased uni-directional current will flow through the rectifier I. In this case, however, none of this current can flow into the condenser 3 which is already charged to a higher potential and it is therefore dissipated in the resistance 2. At the same time the condenser 3 discharges its excess potential through the resistance 2 until the circuit again reaches equilibrium. The inverse resistance of the rectifier I must be high relative to the resistances 9 and 2 or it would not function as a rectifier in this circuit. Therefore no appreciable quantity .of the discharge current of the condenser 3 flows through the rectifier, whence the resistance 9 does not affect the rate of discharge of the condenser. It will thus be apparent that the rate of decrease of potential across the condenser 3 for a given decrease in A. C. input is determined by only two factors,the values of the resistance 2 and the capacity 3-.

The thermionic valve 6 isprovided with any usual means for heating the cathode and is provided with the usual source of plate potential as indicated at I, the output being indicated at 8. Grid bias is provided by any appropriate source of potential in series,-or, if desired, in shunt although this would slightly complicate the calculatlons,-with the uni-directional input circuit above described. In the present instance a battery 5 is shown, around which is shunted the potentiometer 4, one end of which is connected to the input circuit. It will be obvious that the input circuit and the potentiometer may be connected with opposite polarity so asto produce a reversal of the efiect'upon the output.

It will be apparent from the above that I have provided a circuit having an A. C. input and a D. C. output wherein the rate of change of output upon a given increase of input can be varied independently of the rate of change of output upon a corresponding decrease of input, and that these rates may be so different that the circuit responds sufliciently promptly to suchchanges and at the same time is substantially aperiodic.

The resistances 2, 6, and 9 and the capacity 3 are all shown as variable,'but it will be obvious that any or all of these may be fixed if so desired. I find it most convenient to determine the value of the resistance 9 by choosing an appropriate rectifier and to then choose an appropriate size of fixed condenser for 3 to determine the limits of the above values, and to make the resistance 2 variable so as to adjust the relative values within those limits to best suit the particular recording equipment available, but this procedure may be varied to suit the most conveniently available apparatus.

It will be apparent that the resistance 2 in conjunction with the capacity 3 constitutes a filter circuit, and that the used portion of the potentiometer 4 also assists in filtering out from the tube 3 theaudio frequencies applied to the rectifier I. There is therefore little or no audio frequency pulsation in the output of this circuit if the values Figure 2 shows a more practical application of the circuit of Fig. 1. Input, preferably taken directly from the circuit leading to the recorder,

is applied to the winding II of the transformer Il-Ill-IZ. Across the secondary I2 is bridged the potentiometer I3 in order that appropriate amplitudes of potential variation for application to the grid It may be selected by movement of the contact I4. The grid I6 is provided with an appropriate bias by the battery, or equivalent, I9,

and the tube I5 is further provided with an appropriate means, such as the battery 20, for heating its cathode and with the usual plate supply as indicated at 22, through which the plate I! is connected through the impedance 2|, as in the usual impedance coupling. Condenser 23 permits the A. C. component from the plate I! to flow through the primary 25 of the transformer 2524-26, whence it is applied to the grid 29 of the tube 28 across which is bridged the resistance 21 and to which is applied the proper grid bias by the means 33; the tube being provided with a battery 32 to heat the cathode and with a battery 35 to supply potential to the plate 30.

The A. C. component of this tube is, in turn, applied through the condenser 36 and the transformer 38-31-39 t0 the remainder of the circuit which corresponds to the circuit already described in detail under Fig. 1. The secondary 39, of course, corresponds to theinput and the resistor serves to render this. substantially aperiodic. A thermionic rectifier is provided, comprising a cathode heated by a battery 44 and provided with plate current by a battery 58 which also serves to provide the grid bias for the tube 50. The resistance and condenser 46 and 41 correspond to the elements 2 and 3 and the resistance and battery 51 and 58 correspond to the elements 4 and 5. The tube 50 corresponds to the tube 6, the plate current being supplied by. the battery 55, the cathode heating current being supplied by the battery 56 and the output being taken off across the terminals 54. This circuit is one which is capable of commercial use, the

values thereof being as follows: resistance of galvanometer line, two ohms; input impedance,

more than 500 ohms; second stage, stepped down by transformer z-10382; final stage, two we 171As in parallel.

Use is made of the grid voltage vs. plate current characteristic of these tubes to provide a type of operation of the shutter that precludes the possibility of any distortion occurring due to the cutting oif of peaks in the recorded sound.

External supply voltages necessary for operation are 6 volts and 90 volts. A UK 280 tube with the plates connected in parallel is used as the rectifier.

Figure 3 shows the manner in which this amplifier is applied to a sound-recording apparatus, fill representing the amplifier before de-. scribed. The D. C. output of this amplifier is applied to the shutter 6| through the magnetic control member 62, or in any equivalent manner as hereinbefore described, and this serves to control the operation thereof as already set forth. The lines (it and 65 represent the lines to the recorder. A delay circuit, as indicated at 63, may be interposed between the main amplifier and the recorder in order to give the D. C. amplifier time to operate, the constants of the D. C. amplifier being adjusted accordingly as to rate of increase and decrease. This delay circuit may be made in any known fashion, either electrical or mechanical. For example, it may consist of a reproducer and a microphone separated by a mechanical sound-conductor such as a spring or an air column, adjustable to the required time interval; or it may consist of one of the known electrical circuits for such purpose. Such circuit, with appropriate determination of the constants of the control circuits, will pro- .vide theoretically perfect control of the output.

Such theoretically perfect control, is, however, not commercially feasible, and I have accordingly devised circuits which are commercially feasible and at the same time provide a sufficiently close approximation thereto. For ex-- amplifier 69 which controls the shutter 6i through the magnetic control 62. This amplifier 69 is connected back to the amplifier 66 so as to control the output thereof in such a manner as to preclude the output from causing the galvanometer Bl overshooting the sound-track as defined by the shutter. v

The specific manner of connection is better shown'in Fig. 5. In this figure tube 50 corresponds to the tube 50 of Fig. 2, the input, power supply, and output thereof being the same. The screen grid tube ti is connected to an appropriate point in the circuit of the amplifier feeding the galvanometer, and the output of this portion of the amplifier, as indicated at 54', proceeds to the galvanometer. The essential feature of this form of the invention lies in the tube 62 and the manner in which it is connected to the D. C. amplifier. It will be apparent from an inspection of Fig. 5 that any change in the output of the D. C. portion of the amplifier produces, during such change, an output from the transformer 63 whose direction and amplitude depend upon the change applied to the primary of said transformer. It will necessarily follow that if a sudden change occurs in the applied sound, or in other words to the input, any change in the recorded sound will necessarily be delayed until the shutter or equivalent member has had an opportunity to function through the D. C. amplifier already described, thus preventing distortion of the particular variety caused by overshooting the effective portion of the sound track.

A formof the invention which is particularly desirable as well as practicable can be accomplished by the inclusion of a delay circuit in the circuit of Fig. 4, as indicated at 63. In this case the delay need not be nearly as great as when the delay circuit is alone depended on to prevent overshooting when there is an increase of input, with a. corresponding decrease in cost, while the circuit may be adjusted to provide, an appropriate decrease as indicated above, all the details of the circuit having been hereinbefore disclosed.

-It will be obvious to those skilled in the art how to best weigh the factors above given according to the particular recording or other equivalent, apparatus involved and the attendant circumstances as set forth above, wherefore I claim:

1. Sound recording apparatus comprising an amplifier circuit and a ground-noise eliminating circuit, and a delay circuit between the amplifier circuit and the sound recorder.

2. An amplifier having an A. C. input and a D. C. output, including means for determining the rate of change of D. C. output on a decrease in the A. C. input and additional means for determining the rate of change of D. C. output on an increase in A. C. input.

3. An electrical circuit comprising a source of alternating current, a rectifier and a resistance in series therewith, an output circuit of relatively high resistance and a resistance and a capacitance in parallel connected across the circuit between the first resistance and the output circuit.

4. Sound recording apparatus comprising a ground-noise eliminating circuit and an automatic volume control circuit controlled thereby, said ground-noise eliminating circuit comprising an amplifier having an A. C. input and a D. C. output, including means for determining the rate of change of D. C. output on a decrease in the A. C. input and additional means for determining the rate of change of D. C. output on an increase in A. C. input.

5. Sound recording apparatus comprising a ground-noise eliminating circuit and an automatic volume control circuit controlled thereby, said ground noise eliminating circuit including an electrical circuit comprising a source of alternating current, a rectifier and a resistance in series therewith, an output circuit of relatively high resistance, and a resistance and a capacitance in parallel connected across the circuit between the first resistance andthe output circuit.

6. Sound recording apparatus comprising an amplifier circuit and a ground noise eliminating circuit, said ground noise eliminating circuit comprising an electrical circuit having an amplifier having an A. C. input and a D. C. output and including means for determining the rate of change of D. C. output on a decrease in the A. C.

.input and additional means for determining the rate of change of D. C. output on an increase in A. C. input.

7. Sound recording apparatus including an amplifier circuit and a ground noise eliminating circuit comprising an electrical circuit comprising a source of alternating current, a rectifier and a resistance in series therewith, an output circuit of relatively high resistance, and a resistance and a capacitance in parallel connected across the circuit between the first resistance and the output circuit.

BARTON KREUZER. 

